In recent years, along with the widespread use of personal computers, digital cameras capable of easily capturing images have been coming into widespread use. Along with it, there has been demand for more compact digital cameras, and this has naturally led to demand for more compact image sensing optical systems. On the other hand, a year-by-year increasing trend in number of pixels included in image sensing devices has led to demand for image sensing optical systems to have a high optical performance suitable for the increased number of pixels included in image sensing devices and demand for image sensing optical systems to be easy to fabricate to deal with a shorter product life cycle.
Further, various high variable power zoom lens systems having a zoom ratio of over 10× have been proposed as image sensing optical systems for use in video cameras and digital cameras. To meet the demand for more compact and thinner cameras, progress has been being made in size reduction and slimming down of even high variable power zoom lens systems having a zoom ratio of over 10×, by using a technique such as sliding of a lens group, and there is also an expectation for more compact and thinner high variable power zoom lens systems having a zoom ratio of over 20×. To meet these demands, zoom lens systems of various types have been conventionally proposed. For example, Patent Literature 1 listed below proposes a zoom lens system having a zoom ratio of approximately 18× including five lens groups of positive, negative, positive, positive, and positive lens groups, Patent Literature 2 listed below proposes a zoom lens system having a zoom ratio of approximately 20× including four lens groups of positive, negative, positive, and positive lens groups, and Patent Literature 3 listed below proposes a zoom lens system having a zoom ratio of approximately 40× including four lens groups of positive, negative, positive, and positive lens groups.